Digital receivers such as DTT receivers are susceptible to various forms of noise which can disrupt reception. Such receivers are particularly susceptible to impulsive noise, for example caused by domestic electrical appliances and by vehicle ignition systems. Impulsive noise comprises one or more discrete relatively high amplitude pulses of relatively short duration and can disrupt reception to an unacceptable extent. This is discussed in “Impulsive Noise Measurements and Characterisations in a UHF Digital TV Channel”, IEEE Transactions on Electromagnetic Compatibility, Vol. 41, No. 2, May 1999.
In order to attempt to overcome this problem, it is known to perform impulse suppression by zero substitution or “blanking to zero” of the data-carrying signal within a television receiver. In particular, if the noise impulses can be detected, then the data can be blanked to zero so as to attempt to remove the noise energy from the signal, for example supplied to the digital demodulator of the receiver. In order to attempt to detect noise impulses, the absolute value of each digital sample, following analog-to-digital conversion within the receiver, is compared with a predetermined threshold. If a sample exceeds this threshold, it is replaced with zero. Such techniques are disclosed, for example, in “Summary report of some findings on impulsive interference into DVB-T reception”, 10 May 2000, Report published by the BBC Research and Development. However, it has been found that such techniques are of limited use in detecting impulsive noise and do not improve reception sufficiently to avoid undesirable visual artefacts appearing in television pictures.